This invention relates to a method for detecting and healing flaws in the oxidation protective surface coating of a carbon-carbon composite material.
Reinforced composites are used in a wide variety of applications. The best known composites are made from two-dimensional fabrics and/or fibers dispersed in a resin or plastic matrix. These composites are basically a resin or plastic structure to which reinforcing fabrics or fibers have been added to enhance the physical properties of the structure.
Advances in the field of aerospace technology have created a need for high strength, temperature-resistant materials. For many applications, this need is satisfied by carbon-carbon composite materials. These materials utilize a carbon matrix, as opposed to a resin or plastic matrix.
A wide range of multidirectional reinforced composite structures are now available. The simplest of these structures is obtained by stacking unidirectional fibers or sheets with alternating layers oriented in different directions, or by stacking woven sheets. More complex structures provide three-dimensional reinforcement. The simplest of the three-dimensional structures is the three-directional (3D) structure which generally has reinforcing elements which are mutually orthogonal. The most complex three-dimensional structure is a thirteen-directional (13D) structure. The thirteen directions, with reference to a cube, form three subgroups; the three edges, the four long diagonals, and the six diagonals of the faces.
The reinforced carbon-carbon composite structures are fabricated from graphite or carbon yarn or rods. The term yarn includes continuous filament yarns and yarns spun from short fibers, and comprises a plurality of filaments or fibers combined to make up a desired end count. Rods are produced by a pultrusion process whereby unidirectional groups of carbon or graphite yarn are assembled and impregnated with a thermosetting or thermoplastic resin or binder. The impregnated yarn groups are drawn through a die which is warmed to a desired temperature and which has a suitable shape.
The carbon or graphite yarns or rods are assembled into the desired geometric structure. If desired, the yarn may be impregnated with a suitable resin or binder prior to assembly.
The composite is formed either by sintering the reinforcement structure by solidifying the impregnated precursor, thereby avoiding the requirement for other materials, or by the dry or the liquid process, or by a combination of these methods. The dry process consists of depositing pyrolytic carbon inside the structure of the reinforcement by decomposition of a hydrocarbon gas such as methane. In the liquid process, the porous texture of the reinforcement is impregnated with a thermosetting resin or a thermoplastic carbon precursor, such as a phenolic resin, a furanyl resin, coal tar pitch, or the like, that is converted to carbon by heat treatment. Following carbonization, the structure is graphitized. The impregnation, carbonization, graphitization cycle is repeated as often as necessary to densify the composite to a desired degree.
The process of densification of the composite generally comprises heat treatment at a temperature in the range of 2500.degree. to 3000.degree. C. and may include isostatic pressing at pressures up to about 15,000 psi in an oxygen-free environment.
Many applications for carbon-carbon composites have been proposed or implemented. The use of such composites for re-entry heat shield applications has been demonstrated. Ehrenreich, U.S. Pat. No. 3,672,936, discloses disk brake pads made of such composites. The use of these materials for turbine disk and blade components, for propulsion system nozzles, thrust chambers, and ramjet combustion liners, and for re-entry vehicle nosetip applications has been investigated.
In the presence of an inert atmosphere, carbon has a sublimation point in excess of 3500.degree. C. When heated in excess oxygen, carbon burns at about 400.degree. C. For certain applications an allowable amount of wasting away due to combustion can be designed into the structure. For example, a re-entry vehicle heat shield is intended for a useful life of one cycle. For other applications, use and multiple re-use may be desired, in which case wasting away is to be avoided.
Oxidation resistance can be be provided for carbon or graphite materials by depositing silicon over the carbon. Rubisch, U.S. Pat. No. 3,553,010, discloses that flame injection applied silicon reacts with the carbon of the underlying body forming silicon carbide when the operational temperature of the protected parts exceeds about 550.degree. C., which leads to a protective layer of silicon carbide which exhibits oxidation resistance at a relatively high temperature. When heated in the presence of oxygen, the silicon carbide is converted to silicon dioxide which has a very high viscosity when compared to any other glass.
The protective layer of silicon carbide is highly flaw sensitive. If this protective layer is breached, the underlying carbon structure can be quickly oxidized due to the inability of the silica layer to flow and seal the ends of the damaged protective layer. Breach of the silicon carbide layer may occur during densification following application of the silicon carbide layer, as when pressure and/or temperature changes are inadvertently done too rapidly.
The detection and characterization of oxidation initiating defects is particularly difficult in coated C/C materials because the porous nature of both the coating and substrate renders conventional techniques such as fluorescent penetrants or ultrasonics virtually useless. Each pore and crack produces a defect signal and the recognition of the significant defects in this background ranges from exceedingly difficult to impossible. Since the through-the-thickness cracks may be the most important defects that lead to catastrophic oxidation of the substrate and since they occur naturally due to the mismatch in thermal expansions of the substrate and the coating, the healing of these flaws could increase substantially the reliability of coated C/C components.
Accordingly, it is an object of the present invention to provide a method for detecting and healing cracks in the SiC protective coating of a C-C composite.
Other objects and advantages of this invention will be readily apparent from the following description of certain preferred embodiments thereof.